Wire softening apparatus and wire softening method

ABSTRACT

A softening apparatus includes a cooling liquid reservoir for holding a cooling liquid, a first conducting sheave provided outside of the cooling liquid reservoir for applying a voltage to a wire while guiding the wire, and a second conducting sheave provided in the cooling liquid reservoir for letting a current flow through the wire while guiding the wire as the wire is fed via the first conducting sheave. The softening apparatus is also provided with a liquid level detector for detecting a liquid level of the cooling liquid in the cooling liquid reservoir. A heating path length of the wire between the first conducting sheave and the second conducting sheave is controlled based on a result of detection by the liquid level detector.

This application is the national phase of PCT International Application No. PCT/JP2012/055032 that has an International filing date of Feb. 29, 2012 and designated the United States of America and claims priority to Japanese Patent App. No. JP 2011-232513 that was filed on Oct. 24, 2011. The disclosure of the prior applications is hereby incorporated by reference herein in their entirety.

BACKGROUND

In the automobile industry and other industries, electric wires consisting, for example, of stranded annealed copper wires are used as wiring material for electrical communication and other purposes. To connect an electric wire to electric equipment, the terminal crimped on one end of the electric wire is inserted into a connector, which then is interconnected with a connector of the electric equipment.

Annealed copper wires, such as those mentioned above, are manufactured through processes of drawing out wires, thermally softening (annealing) the drawn-out wires, etc. JP H08-92778A discloses a technology for thermally softening wires.

According to JP H08-92778A, a wire is Joule heated while passing through a heating zone between two power feed rolls and then cooled underwater in a cooling zone.

SUMMARY Technical Problem

When crimping a terminal to an end of a wire, it is necessary to ensure a certain level of crimp strength between the wire and the terminal. However, if there is a large variation in crimp strength between wires and terminals, it is difficult to ensure a certain level of crimp strength between the wires and the terminals.

Accordingly, the variation in crimp strength is ideally limited between wires and terminals.

Solution to Problem

In a first aspect, a wire softening apparatus comprises a cooling liquid reservoir for holding a cooling liquid, a first conducting sheave provided outside of the cooling liquid reservoir for applying a voltage to a wire while guiding the wire, a second conducting sheave provided in the cooling liquid reservoir for letting a current flow through the wire while guiding the wire as the wire is fed via the first conducting sheave, a liquid level detector for detecting a liquid level of the cooling liquid in the cooling liquid reservoir, and a heating path length controller for controlling a heating path length of the wire between the first conducting sheave and the second conducting sheave based on a result of detection by the liquid level detector.

A second aspect is a wire softening apparatus according to the first aspect, wherein the heating path length controller includes a liquid level adjuster for adjusting the liquid level of the cooling liquid in the cooling liquid reservoir, and wherein the adjustment by the liquid level adjuster is controlled based on the result of detection by the liquid level detector to maintain the liquid level of the cooling liquid in the cooling liquid reservoir within a certain range.

A third aspect is a wire softening apparatus according to the first aspect, wherein the heating path length controller includes a sheave position adjuster for adjusting the position of the first conducting sheave relative to the second conducting sheave, and wherein the adjustment of the position of the first conducting sheave by the sheave position adjuster is controlled based on the result of detection by the liquid level detector to maintain the distance between the first conducting sheave and the liquid level of the cooling liquid in the cooling liquid reservoir within a certain range.

In a fourth aspect, a method for softening a wire comprises (a) feeding the wire from a first conducting sheave provided outside of a cooling liquid reservoir to the first conducting sheave provided in the cooling liquid reservoir while letting a current flow through the wire between the first conducting sheave and the second conducting sheave, (b) cooling the wire with cooling water in the cooling liquid reservoir, and (c) controlling a heating path length of the wire between the first conducting sheave and the second conducting sheave based on a fluctuation in a liquid level of the cooling liquid in the cooling liquid reservoir.

Advantageous Effects

According to the method for softening a wire of the first aspect, the variation in the heating path length of the wire between the first conducting sheave and the second conducting sheave can be limited based on a result of detection by the liquid level detector to control the heating path length, thereby limiting the variation in crimp strength between wires and terminals.

According to the second aspect, the variation in the heating path length can be limited by maintaining the liquid level of the cooling liquid in the cooling liquid reservoir within a certain range.

According to the second aspect, the variation in the heating path length can be limited by maintaining the distance between the first conducting sheave and the liquid level of the cooling liquid in the cooling liquid reservoir within a certain range.

According to the method for softening a wire of the fourth aspect, the variation in the heating path length of the wire between the first conducting sheave and the second conducting sheave can be limited based on a fluctuation in a liquid level of the cooling liquid in the cooling liquid reservoir to control the heating path length, thereby limiting the variation in crimp strength between wires and terminals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an apparatus for softening a wire W according to related technology.

FIG. 2 is a diagram showing the relationship between the heating conditions (softening temperature×heating period) and the breaking load and the breaking elongation.

FIG. 3 is a diagram showing the relationship between the liquid level of cooling liquid, the temperature of the cooling liquid, and a nitrogen content and the breaking load.

FIG. 4 is a diagram sorting FIG. 3 by the levels of each factor.

FIG. 5 is a schematic diagram showing a wire softening apparatus according to a first embodiment.

FIG. 6 is a flowchart showing the process performed by the foregoing wire softening apparatus.

FIG. 7 is a schematic diagram showing a wire softening apparatus according to a second embodiment.

FIG. 8 is a flowchart showing the process performed by the foregoing wire softening apparatus.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following describes wire softening apparatuses and wire softening methods according to certain embodiments.

Background

The background of related technology is described first. FIG. 1 is a schematic diagram of an apparatus 100 for softening a wire W according to the background technology.

The apparatus 100 for softening the wire W includes a water tank 110, a first conducting sheave 120, and a second conducting sheave 122.

The water tank 110 holds cooling liquid 112. The water tank 110 is initially supplied with liquid by an operator or other personnel until a predetermined liquid level is reached.

The first conducting sheave 120 and the second conducting sheave 122 are annular members capable of carrying the wire W. The second conducting sheave 122 is provided in the water tank 110, and the first conducting sheave 120 is provided outside of the water tank 110 above the second conducting sheave 122.

After the drawn or otherwise processed wire W is submerged in the cooling liquid 112 in the water tank 110 while running over the first conducting sheave 120 and the second conducting sheave 122, the wire W leaves the water tank 110 to be subsequently wound up.

A heating power source is connected to the first conducting sheave 120 and the second conducting sheave 122 to apply a voltage to the wire W to let a current flow through the wire between the first conducting sheave 120 and the second conducting sheave 122.

In addition, a cylindrical member 130 is provided to surround the wire W where it runs between the first conducting sheave 120 and the second conducting sheave 122. The lower end of the cylindrical member 130 is submerged in the cooling liquid 112. Nitrogen is constantly supplied into the cylindrical member 130 so that the space inside the cylindrical member 130 is always filled with nitrogen. This nitrogen serves to inhibit oxidation of the wire W during heating.

The wire W is softened by Joule heating between the first conducting sheave 120 and the second conducting sheave 122. The wire W thus heated is also cooled by being submerged in the cooling liquid 112 to be subsequently wound up. A wire W or annealed copper wire for use as the core of an electric cable or the like is manufactured in this way.

A plurality of wires W is thus manufactured by such a softening process and are twisted together to make an electric wire with the twisted wires as the core. When crimping a terminal to this electric wire, it is necessary to ensure a certain level of crimp strength between the wire and the terminal. From the viewpoint of limiting the variation in crimp strength between wires and terminals, it was intended to ensure a certain level of crimp strength between the wires and the terminals.

In light of the positive correlation between the crimp strength of the wire to the terminal and the breaking load of the wire W, limiting the variation in the breaking load was considered. First, the relationship between the heating conditions (softening temperature×heating period) and the breaking load and the breaking elongation was explored. As used herein, the breaking load refers to the load (N) at which the wire W breaks when pulled in its longitudinal direction, and the breaking elongation refers to the permanent elongation when the wire W breaks as it is pulled in its longitudinal direction (the ratio in percentage (%) of the amount of elongation to the original length).

Assuming that the foregoing wire softening apparatus 100 is used, the softening temperature can be theoretically determined by the equations set forth below.

First, the electric energy [WH] applied to the wire W between the first conducting sheave 120 and the second conducting sheave 122 is determined by the equation below, where S [m] is the distance between the first conducting sheave 120 and the liquid level of the cooling liquid 112, V1 [m/min] is the velocity of the wire W, V [V] is the voltage applied between the first conducting sheave 120 and the second conducting sheave 122, and I [A] is the current flowing through the wire W.

W[WH]={S[m]/V1 [m/min]/60[min/H]}×V[V]×I[A]  (Eq. 1)

Also, the amount of heat Q [cal] applied to the wire W between the first conducting sheave 120 and the second conducting sheave 122 is expressed as follows:

Q [cal]=W[WH]×0.2389 [cal/W sec]×3600 [sec/H]  (Eq. 2)

Also, if the ambient temperature is T1 [° C.], if the softening temperature is T2 [° C.], if the specific heat of the wire W is c [cal/g·° C.], and if the weight of the wire W is m [g], then the foregoing amount of heat Q [cal] is given as follows:

Q [cal]=m[g]×c [cal/g·° C.]×(T2 [° C.]−T1 [° C.])  (Eq. 3)

It is therefore possible to determine Q [cal] from Equations 1 and 2, which in turn can be substituted into Equation 3 to determine the theoretical softening temperature T2 [° C.].

Given the foregoing conditions, if the wire W is softened by the softening apparatus 100 shown in FIG. 1, the relationship between the heating conditions (softening temperature×heating period, on the horizontal axis of FIG. 2) and the breaking load (on the vertical axis of FIG. 2) and the breaking elongation is as shown in FIG. 2. As can be seen from FIG. 2, the change in the heating conditions greatly affects the breaking load. It therefore appears that it is preferable to limit the variation in the heating conditions if the variation in the breaking load is to be limited.

The range of the heating conditions where the breaking load greatly changes largely overlaps the range of the heating conditions where the breaking elongation greatly changes. Furthermore, the greater the heating conditions are, the smaller the breaking load is and the greater the breaking elongation is. For this reason, it is also preferable to limit the variation in the heating conditions in order to maximize the breaking load and the breaking elongation at the same time.

Then, experimentations were conducted to determine how much the variation in the heating conditions affects the variation in the breaking load when using the foregoing softening apparatus 100.

It was first determined what caused the variation in the heating conditions in the foregoing softening apparatus 100. It is assumed that the velocity V1 of the wire W, the applied voltage V [V], the current I [A] can be kept largely constant in the foregoing softening apparatus 100. The amount of cooling liquid 112 in the water tank 110, however, decreases due to evaporation of the cooling liquid 112 by heat, adhesion of the cooling liquid 112 to the wire W, etc. As the wire W is apparently heated between the first conducting sheave 120 and the liquid level of the cooling liquid 112, the fluctuation in the liquid level of the cooling liquid 112 changes the heating path length S of the wire W, thus affecting the period of heating. Based on the above, it was assumed that the variation in the heating conditions is largely attributable to the fluctuation in the liquid level of the cooling liquid 112.

Accordingly, the liquid level of the cooling liquid 112 was used as a factor, i.e., the liquid level was changed between standard (a predetermined standard level), intermediate (two centimeters below the standard level), and low (four centimeters below the standard level) to determine how the breaking load is affected.

As additional factors, the temperature of the cooling liquid 112 was changed between high and low, and the nitrogen content was also changed between high, standard, and low to determine how the breaking load is affected.

According to an experimental design, experiments were conducted by selecting combinations of the liquid level (position) of the cooling liquid 112, the temperature of the cooling liquid 112, and the nitrogen content, which produced the results shown in FIG. 3. Note that each of the experimental results in FIG. 3 shows the average value of the breaking load obtained from three experiments. FIG. 4 shows these results sorted by the levels of each factor.

It was determined from FIG. 4 that the fluctuation in the liquid level of the cooling liquid 112 more greatly affects the breaking load than the change in the temperature of the cooling liquid 112 or the change in the nitrogen content.

Based on the above, it was discovered that it is preferable to maintain the liquid level within a certain range in the softening apparatus 100 shown in FIG. 1 in order to limit the variation in the crimp strength of wires to terminals, i.e., the variation in the breaking load. Moreover, it was found that, as the fluctuation in the liquid level changes the heating path length S of the wire W, ultimately, it is the heating path length S of the wire W that should be maintained within a certain range.

Wire Softening Apparatuses and Wire Softening Methods First Embodiment

Described first is an apparatus 10 for softening a wire W according to a first embodiment. FIG. 5 is a schematic diagram showing the apparatus 10 for softening the wire W. The softening apparatus 10 includes a cooling liquid reservoir 20, a first conducting sheave 30, a second conducting sheave 32, a liquid level detector 40, and a heating path length controller 50.

The cooling liquid reservoir 20 is formed as an upwardly open vessel that holds cooling liquid 22 therein. Liquid such as water is used as the cooling liquid 22.

The first conducting sheave 30 and the second conducting sheave 32 are annular members capable of carrying the wire W. The second conducting sheave 122 is provided in the cooling liquid reservoir 20, and the first conducting sheave 30 is provided outside of the cooling liquid reservoir 20 vertically above the second conducting sheave 122. Here, considering the distance required to thermally soften the wire W, the second conducting sheave 122 is disposed in an upper position well above the liquid level of the cooling liquid 22.

After the drawn or otherwise processed wire W is submerged in the cooling liquid 22 in the cooling liquid reservoir 20 while being guided by the first conducting sheave 30 and the second conducting sheave 32, the wire W leaves the cooling liquid reservoir 20 to be subsequently wound up.

Additionally, a heating power source is connected to the first conducting sheave 30 and the second conducting sheave 32 to apply a voltage for letting a current flow through the wire W between the first conducting sheave 30 and the second conducting sheave 32.

In addition, a cylindrical member 38 is provided to surround the wire W where it runs between the first conducting sheave 30 and the second conducting sheave 32. The lower end of the cylindrical member 38 is submerged in the cooling liquid 22. Nitrogen is constantly supplied into the cylindrical member 38 so that the space inside the cylindrical member 38 is always filled with nitrogen. This nitrogen serves to inhibit oxidation of the wire W during heating.

The wire W is softened by Joule heating between the first conducting sheave 30 and the second conducting sheave 32. The wire W thus heated is also cooled by being submerged in the cooling liquid 22, and is subsequently wound up.

The softening apparatus 10 includes the liquid level detector 40 and the heating path length controller 50 to limit the variation in the heating path length S of the wire W.

The liquid level detector 40 is configured to detect the liquid level of the cooling liquid 22 in the cooling liquid reservoir 20. As the liquid level detector 40, an optical or ultrasonic sensor utilizing reflection on the liquid surface or an electrode type detector based on the conduction obtained by submerging the electrodes in the liquid will suffice. Alternatively, a detector in which the liquid level is detected by the level of a floater on the liquid surface will also serve the purpose.

The heating path length controller 50 is configured to control the heating path length controller 50 for the wire W between the first conducting sheave 30 and the second conducting sheave 32 based on the result of detection by the liquid level detector 40. Here, although a current flows through the wire W between the first conducting sheave 30 and the second conducting sheave 32, the temperature of the part submerged in the cooling liquid 22 becomes sufficiently low. Accordingly, it is believed that the wire W is thermally softened (annealed) between the first conducting sheave 30 and the liquid surface of the cooling liquid 22. That is to say, the heating path length S of the wire W is determined by the distance between the first conducting sheave 30 and the liquid surface of the cooling liquid 22. Accordingly, to limit the variation in the heating path length S, the liquid level needs to be adjusted to remain within a certain range, or the position of the first conducting sheave 30 needs to be adjusted.

The heating path length controller 50 includes a liquid level adjuster 52 and a control unit 60.

The liquid level adjuster 52 is configured to adjust the liquid level of the cooling liquid 22 in the cooling liquid reservoir 20 based on the result of detection by the liquid level detector 40. Particularly, the liquid level adjuster 52 includes a water supply pipe 54 connected to a cooling liquid source (e.g. a tank) (not shown) and a water supply pump 56 installed midway in the water supply pipe 54. The inlet of the water supply pipe 54 is positioned to enable supplying cooling liquid 22 into the cooling liquid reservoir 20. Upon activation, the water supply pump 56 can supply the cooling liquid 22 into the cooling liquid reservoir 20. Note that if pressure is applied to the cooling liquid 22 flowing through the water supply pipe 54 for feeding the cooling liquid, the water supply pump 56 can be replaced with a magnetic valve or the like. In any case, the liquid level adjuster 52 is configured to adjust the liquid level of the cooling liquid 22 in the cooling liquid reservoir 20 by supplying the cooling liquid 22 to the cooling liquid reservoir 20.

Note that the liquid level adjuster 52 may also include a drain pipe for draining the cooling liquid 22 out of the cooling liquid reservoir 20 and a magnetic valve or the like, installed in the drain pipe. This allows excess cooling liquid 22 supplied into the cooling liquid reservoir 20 to be drained away. However, while feeding the wire W, the cooling liquid 22 will generally only decrease due to evaporation or adhesion of the cooling liquid 112 to the wire W. Accordingly, no problem will arise if such a draining arrangement is omitted.

Note that the cooling liquid reservoir 20 need not necessarily be replenished with cooling liquid 22 to maintain the liquid level of the cooling liquid 22 within a certain range. For example, the capacity of the cooling liquid reservoir 20 may be adjusted by submerging a separate object in the cooling liquid reservoir 20.

Based on the result of detection by the liquid level detector 40, the control unit 60 controls the adjustment by the liquid level adjuster 52 to maintain the liquid level of the cooling liquid in the cooling liquid reservoir 20 within a certain range.

The control unit 60 includes a microprocessor, a main memory, and an auxiliary memory connected to the microprocessor. The main memory is comprised of a RAM (Random Access Memory), etc., and the auxiliary memory is comprised of a nontemporary storage device, such as a flash memory, an EPROM (Erasable Programmable ROM), a hard disk device, etc. The auxiliary memory stores a program describing instructions for the microcomputer. The microprocessor controls the adjustment by the liquid level adjuster 52 by reading the program and carrying out the process steps described below.

FIG. 6 is a flowchart showing the process carried out by the control unit 60.

First, at Step S1, the liquid level is obtained via the liquid level detector 40.

Then, at Step S2, it is determined whether or not the liquid level is lower than a first reference level. The first reference level is a value preset by an operator or other personnel and stored in the auxiliary memory of the control unit 60. The first reference level is a value set according to a thermal softening condition range (a range of the heating path length S) suitable for obtaining the target breaking load, and preferably, it is set as a position that corresponds to the upper limit of the range of the heating path length S. The thermal softening condition range (the range of the heating path length S) itself is determined experimentally or empirically according to the required breaking load, etc. If the result is YES at Step S2, the process advances to Step S3, and if the result is NO, the process advances to Step S4. Note that if the liquid level is the same as the first reference level, the process may go to either Step S3 or S4.

At Step S3, the control unit 60 gives an ON command to the water supply pump 56. This activates the water supply pump 56 to start feeding cooling liquid 22 to the cooling liquid reservoir 20. As the cooling liquid 22 is supplied, the liquid level of the cooling liquid 22 rises in the cooling liquid reservoir 20. Subsequently, the process returns to Step S1.

If, on the other hand, it is determined at Step S2 that the liquid level is not lower than the first reference level, the process goes to Step S4. At Step S4, it is determined whether the liquid level is higher than a second reference level. The second reference level is also a value preset by an operator or other personnel and stored in the auxiliary memory of the control unit 60. The second reference level is a value set according to a thermal softening condition range (a range of the heating path length S) suitable for obtaining the target breaking load, and preferably, it is set as a position that corresponds to the lower limit of the range of the heating path length S. The second reference level may be the same as the first reference level. Then, if the result is YES at Step S4, the process advances to Step S5, and if the result is NO, the process returns to Step S1. Note that if the liquid level is the same as the second reference level, the process may go to either Step S4 or S5.

At Step S5, an OFF command is issued to the water supply pump 56. This causes the water supply pump 56 to stop feeding the cooling liquid 22. Subsequently, the process returns to Step S1.

By repeating the foregoing process, the liquid level of the cooling liquid 22 in the cooling liquid reservoir 20 is maintained between the first reference level and the second reference level.

Note that part or all of the control unit 60 may be implemented in hardware. In other words, any arrangement will suffice as the control unit 60 as long as it can control the operation of the liquid level adjuster 52 according to the result of detection by the liquid level detector 40.

Also, for example, if the liquid level detector is configured to output an ON signal (or an OFF signal) under normal operating conditions and output an OFF signal (or an ON signal) when the liquid level falls below a predetermined value, the operation of the water supply pump 56 may be controlled based on a signal corresponding to the OFF signal (or the ON signal) from the liquid level detector.

According to the wire softening apparatus 10 thus constructed and the method of softening a wire W thereby, the liquid level of the cooling liquid 22 in the cooling liquid reservoir 20 is maintained between the first reference level and the second reference level. This allows the distance between the first conducting sheave 30 and the liquid level of the cooling liquid 22, i.e., the heating path length S of the wire W between the first conducting sheave 30 and the second conducting sheave 32, to be maintained within a certain range. This limits the variation in the heating path length S of the wire W, thus limiting the variation in the breaking load, i.e., the crimp strength between wires and terminals.

Second Embodiment

The following describes an apparatus 10B for softening a wire W according to a second embodiment. In the following description, components identical to those described with respect to the first embodiment are assigned identical reference numerals and their further description is omitted. FIG. 7 is a schematic diagram showing an apparatus 10B for softening the wire W. The softening apparatus 10B includes a cooling liquid reservoir 20, a first conducting sheave 30, a second conducting sheave 32, a liquid level detector 40, and a heating path length controller 50B.

The cooling liquid reservoir 20, the first conducting sheave 30, the second conducting sheave 32, and the liquid level detector 40 are identical with those described with respect to the first embodiment.

The heating path length controller 50B includes a sheave position adjuster 70 for adjusting the position of the first conducting sheave 30 relative to the second conducting sheave 32 and a control unit 60B. The sheave position adjuster 70 comprises a linear motor or the like, which is capable of linear position controlling and disposed above the first conducting sheave 30. It is also configured to support the second conducting sheave 32 in a manner that can vertically move and adjust the position of the second conducting sheave 32 above the first conducting sheave 30.

The control unit 60B is configured to control the operation of the sheave position adjuster 70 to adjust the position of the first conducting sheave 30 based on the result of detection by the liquid level detector 40 in order to maintain the distance between the first conducting sheave 30 and the liquid level of the cooling liquid 22 in the cooling liquid reservoir 20 within a certain range.

Similar to the foregoing control unit 60, the control unit 60B is implemented by a configuration including a microprocessor, a main memory and an auxiliary memory connected to the microprocessor.

FIG. 8 is a flowchart showing the process carried out by the control unit 60B.

First, at Step S11, the liquid level is obtained via the liquid level detector 40. Here, the liquid level is obtained as a continuous or discrete value.

In the following Step S12, a command regarding the position of the first conducting sheave 30 is issued to the sheave position adjuster 70 according to the obtained liquid level, thereby causing the first conducting sheave 30 to move to the position corresponding to the command. The position command to the sheave position adjuster 70 is for realizing a thermal softening condition range (a range of the heating path length S) suitable for obtaining the target breaking load. This position is calculated, for example, as the difference between the obtained liquid level and a proper liquid level that is preset as being suitable for obtaining the target breaking load. The first conducting sheave 30 is moved to the position relative to the liquid level corresponding to the desirable heating path length S. Subsequently, the process returns to Step S11.

By repeating the foregoing process, the liquid level of the cooling liquid 22 and the first conducting sheave 30 is maintained within a certain range.

Note that part or all of the control unit 60B may be implemented in hardware. That is, any arrangement will suffice as the control unit 60B as long as it can control the operation of the sheave position adjuster 70 according to the result of detection by the liquid level detector 40.

According to the apparatus 10B and the method for softening the wire W thereby, the distance between the first conducting sheave 30 and the liquid level of the cooling liquid 22 is maintained within a certain range by moving the first conducting sheave 30 according to the fluctuation in the liquid level. This maintains the heating path length S of the wire W between the first conducting sheave 30 and the second conducting sheave 32 within a certain range and, as above, limits the variation in the heating path length S of the wire W, thus limiting the variation in the breaking load, i.e., the crimp strength between wires and terminals.

Variants

Note that the elements described with respect to the foregoing embodiments and variants may be combined as required as long as they are compatible with each other.

Having described the present invention in detail, the foregoing description is illustrative in all aspects and the present invention is not limited thereto. It is understood that countless variants not illustrated herein are conceivable without deviating from the scope of the present invention. 

1. A wire softening apparatus, comprising: a cooling liquid reservoir configured to hold a cooling liquid; a first conducting sheave provided outside of the cooling liquid reservoir configured to apply a voltage to a wire while guiding the wire; a second conducting sheave provided in the cooling liquid reservoir configured to let a current flow through the wire while guiding the wire as the wire is fed via the first conducting sheave; a liquid level detector configured to detect a liquid level of the cooling liquid in the cooling liquid reservoir; and a heating path length controller configured to control a heating path length of the wire between the first conducting sheave and the second conducting sheave based on a result of detection by the liquid level detector.
 2. The wire softening apparatus according to claim 1, wherein the heating path length controller includes: a liquid level adjuster configured to adjust the liquid level of the cooling liquid in the cooling liquid reservoir, and wherein the adjustment by the liquid level adjuster is controlled based on the result of detection by the liquid level detector to maintain the liquid level of the cooling liquid in the cooling liquid reservoir within a certain range.
 3. The wire softening apparatus according to claim 1, wherein the heating path length controller includes: a sheave position adjuster configured to adjust the position of the first conducting sheave relative to the second conducting sheave, and wherein the adjustment of the position of the first conducting sheave by the sheave position adjuster is controlled based on the result of detection by the liquid level detector to maintain the distance between the first conducting sheave and the liquid level of the cooling liquid in the cooling liquid reservoir within a certain range.
 4. A method for softening a wire, comprising the steps of: (a) feeding the wire from a first conducting sheave provided outside of a cooling liquid reservoir to the first conducting sheave provided in the cooling liquid reservoir while letting a current flow through the wire between the first conducting sheave and the second conducting sheave; (b) cooling the wire with cooling water in the cooling liquid reservoir; and (c) controlling a heating path length of the wire between the first conducting sheave and the second conducting sheave based on a fluctuation in a liquid level of the cooling liquid in the cooling liquid reservoir. 